Changes afoot on Ordnance Survey paper maps and a useful reminder of the importance of Datums!

You may never have noticed, but there are feint markings along the margins of Ordnance Survey paper maps. These represent not the OS grid with which we are familiar and most spatial references in the UK still use (eg. NT 123 456) but rather a latitude-longitude grid.  This system is more international than the GB grid, which is just effective over the local area of our small islands.  The current lat-long grid uses the Airy 1830 ellipsoid/datum, and the OS are proposing replacing this with the WGS84 datum to give compatibility with the GPS system.  This reminds us that lat-long references (or indeed any grid-based spatial reference) is only unique when its datum is specified. Its no use just quoting:

55.9294° N,  3.1642° W

without the datum that could represent different places.

So what does Ordnance Survey’s change mean?  Well it means any lat-long references taken from the paper map (probably fairly unlikely) will now be wrong – actually not wrong just in need of conversion to the different datum. The OS say that for “Landranger Maps, this movement may be as little as 2mm.”  That’s 100m on the ground.  They don’t give a maximum error. Further details are here


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